This invention relates to a semiconductor memory device, and more particularly to a redundant circuit in such semiconductor memory device.
Recently, an internally equipped redundant circuit has been demanded to increase the yield of a semiconductor device. This will soon also become necessary for a PROM and a mask ROM. Since a mask ROM cannot use a spare cell for its redundant circuit, a PROM is used as a substitute for the spare cell.
This redundant circuit is used when the main cell is faulty, and a higher yield is expected if the device is equipped with more than one redundant circuit. In this example, the device is assumed to be equipped with two redundant circuits.
When it is equipped with two redundant circuits, the semiconductor memory device can still operate when the main cell has one or two failures. When the main cell has no failure, both redundant circuits are in the not-in-use status. When it has one failure, one of the two redundant circuits is put into the in-use status while the other is left in the not-in-use status. When it has two failures, both redundant circuits are put into the in-use status. In all of these cases, the semiconductor memory device is operable.
Since failures occur randomly, they may occur in the main cell or in the redundant circuits. If a failure occurs in one of the redundant circuits, there is no problem, providing a failure does not occur simultaneously in the main cell, since in this case neither of the redundant circuits would be put into the in-use status anyway. If a failure occurs simultaneously in the main cell, a problem may arise, since the failed redundant circuit may be put into the in-use status. However, a problem does not arise if the non-failing circuit is put into the in-use status.
Thus, when a failure occurs in the main cell, the failed circuit is put into the in-use status and the non-failing circuit is put into the not-in-use status, the failure of the main cell is not cured and the device remains faulty.
Therefore, it is desirable to prohibit the use of a failed redundant circuit and to put the non-failing circuit into the in-use status.